The present invention relates to a joint prosthesis, sometimes termed a surface replacement, for the distal radioulnar (DRU) joint of the forearm.
Cartilage destruction of the distal radioulnar joint is often caused by disease, such as different types of inflammatory diseases, especially rheumatoid arthritis. Today these injuries are frequently operated rather late in the evolution of the disease when pain evolves or mobility starts to decrease. At this time, the joint is most often destroyed, with little remaining cartilage and with varying degrees of bone destruction. A operation commonly used is the Darrach procedure, which consists of a simple resection of the ulnar head (caput ulna). The cut ulnar bone-end is now mobile and xe2x80x9cfloatsxe2x80x9d and sometimes the wrist feels unstable and painful. There is a risk for the ulna and radius to stick to each other. Sometimes the patient feels a clicking sensation, sometimes painful, when turning the forearm. Another potential consequence of rheumatoid arthritis is destruction of the ligaments, joint capsule or other connective tissue stabilizers crossing the DRU joint. A tear or weakening of these structures, such as the distal radioulnar ligaments and the interosseus membrane, as a result of rheumatoid diseases can also compromise the stability of the DRU joint because of the loss of tension in the radioulnar ligaments. Such loss of ligament tension may allow the DRU joint to sublux or dislocate.
In non-rheumatoid patients, the DRU joint may often be injured as a consequence of a distal radial fracture and by a tear in the distal radioulnar ligaments or interosseus membrane, causing a secondary joint surface incongruity or instability of the distal radial ulnar joint. Such an incongruity may also occur as a consequence of an intraarticular radial fracture extending into the DRU joint; as a result, the joint surface may heal with a step-off causing an incongruity. Also a radial fracture, which does not extend into the DRU joint, might influence the congruity due to an angulation of the radial shaft and the radial joint surface of the DRU joint. A distal radioulnar ligament tear might also compromise the stability of the DRU joint as a consequence of loss of tension in the radioulnar ligaments or interosseus membrane, and such a loss of ligament tension may allow the DRU joint to sublux or dislocate.
The consequence of an incongruity may be an osteoarthritis, which may be symptomatic. Different treatment alternatives exist, none of them being particularly good. All are compromises, trading different wrist and hand functions to achieve pain relief. A common method is the Sauvee-Kapandjii procedure, where the ligaments from the ulnar tip to the radius and carpus are maintained, the ulna is resected proximally and screws are used to keep the ulnar head in contact with the radius. The radius together with the ulnar head then pivot, within the osteotomy defect. Other known methods include the Bowers hemiresection of the ulnar end with soft tissue interposition and the Watson distal ulnar resection. Another method resects the ulnar head and replaces it with a prosthesis.
An object of the present invention is to develop a device to make it possible to use a method of operation which would lead to better clinical results than the various methods in use today.
Another object of the invention is to make it possible to keep the DRU joint as intact as possible through a device, that allows resurfacing the articular surfaces of the distal portion(s) of radius and/or ulna bone(s) that form the DRU joint without disturbing the ligaments and their attachment sites that stabilize the DRU joint.
It is also an object of the invention to make it possible to stabilize an unstable DRU joint by providing a means to adjust the tension, generally to increase it, of the distal radioulnar ligaments and the interosseus membrane. A surgeon, after having prepared the implantation site, will be able to adjust the tension of the stabilizing structures, such as the distal radioulnar ligaments or interosseus membrane. This can be done by increasing or decreasing the distance between the radius and ulna by selecting component(s) of appropriate size and thickness.
It is a further object of the invention to make it possible to employ only the radial or ulnar component of the device (sometimes termed a hemi implantation), to resurface only the articular surface of the distal radius or the distal ulna respectively, if the clinical findings are such that the destruction of the articular surfaces of the DRU joint is confined exclusively to either the radius or the ulna. The device, i.e. the radial or ulnar component, when used as hemi implant, allows adjustment of the tension of the distal radioulnar ligament by selecting an appropriate size device.
Yet another object of the invention is to provide a means to resurface two distinct types of ulnar DRU joint cartilage damage. One embodiment of the ulnar component of the present invention provides a means to resurface only the distal lateral region of the ulna that is the site of articular cartilage in a physiologic DRU joint. This distal lateral region of the distal ulna articulates against the ulnar notch of the radius. A second embodiment of the ulnar component of the present invention provides a means to resurface both the distal lateral region of the ulna and the portion of the distal end of the ulna facing the TFCC and the lunate bone, which are sites where there is articular cartilage.
A device according to the invention can be used following a radial fracture, or the onset of rheumatoid arthritis or other rheumatoid disease involving the distal radioulnar joint. The prosthesis generally supports the turning (pronation/supination) motion of the forearm. The ulna is the non-moving and weight-bearing fundament of the distal radioulnar joint, while the radius is the mobile component, with mostly compressive forces influencing its positioning during its turning movement. The radius turns around the ulnar head. Besides the distal joint surfaces between the radius and ulna, a prerequisite for the turning movement is the existence of joint surfaces proximally in the elbow. The proximal radioulnar joint consists of the radial head and the ulna with a fossa radii and an annular ligament.
In the elbow, the ulna makes a flexion or extension motion, whereas the radius rotates around an axis which passes through the proximal head of the radius and the distal head of the ulna. Both the radius and the ulna have such a curvature that the middle points of the two bones are fairly far away from each other. Through this arrangement, the radius has enough space to be able to rotate around the ulna.
Stability depends both on the congruity of the two radioulnar joints as well as on the ligaments of the two joints keeping the radius and ulna together. Muscles forces push the two bones together, still allowing them to make both a rolling and a translatory motion relative each other. The forces in the distal radioulnar joint thus are mainly compressive. Different parts of the ulnar head will be in contact with the joint surface of the radius as the ulnar head successively translates and rotates along the joint surface. In the two extreme positions of pronation and supination, the joint surface of the radius is loaded in the volar part and the dorsal part respectively. The distal radioulnar joint ligaments stop the joint from luxation, and secondarily, the joint is stabilized by the interosseus membrane
When problems arise in the distal radioulnar joint, it is common, as previously mentioned, to simply resect the ulnar head and attach it to the radius or to replace it with a prosthesis. The result of such treatment is often that the distal position of the radius becomes changed (because the radius is resting upon the ulna) and such a change of position may often makes the result of the operation less than satisfactory.
In contrast, through an implantation of a surface replacement according the present invention, the ulna is kept intact; thus it is able to support the radius without changing the position of the radius in an unfavourable way.
Generally surface replacements embodying various features of the invention make it possible to achieve the above-mentioned objects and demands. Moreover, the invention makes it possible to preserve the ligamentous apparatus when implanting a surface replacement. This means that the compressive forces within the distal radioulnar joint are maintained after the operation and contribute to the stabilization of the joint. By using appropriate of the different component sizes which are available, the invention further makes it possible, in cases where certain stabilizing ligaments are destroyed by disease or injury, to adjust the tension of the remaining stabilizing structures in order to get a stabile joint.